Conference Agenda

In this paper, we introduce a secure optical communication protocol that harnesses quantum correlation within entangled photon pairs. A message written by acting on one of the photons can be read exclusively through measurements of the other photon of the pair. In this scheme, a bright, meaningless optical beam hides the message, rendering it inaccessible to potential eavesdroppers. Unlike traditional methods, our approach only affects unauthorized users, fundamentally limiting their access to the communication channel. We demonstrate the effectiveness of our protocol by achieving secure communication through both amplitude and phase modulation, relying on single-photon measurements, as opposed to most approaches which rely on coincidence measurements. We successfully demonstrate the resilience of the data transfer to noise up to 10^5 times greater than the signal, and we employ this technique for the secure transfer of an image.

We develop a framework that computes two-photon spontaneous emission (TPSE) spectra of a quantum emitter near an arbitrarily shaped nanostructure. The model considers the interaction up to the electric quadrupolar order, which is relevant for nanophotonic structures sustaining strongly confined fields that are used to enhance and to tailor spontaneous emission processes. Moreover, we consider interference effects between multipolar two-photon emission channels, for the first time to our knowledge. First, we show for a s → s transition of a hydrogen atom placed under a silver plasmonic nanodisk a substantial enhancement in the photon-pair emission rates by 5 and 11 orders of magnitude for the two-electric dipole (2ED) and two-electric quadrupole (2EQ) transitions, respectively. Then for the same emitter under a plasmonic graphene nanotriangle, we demonstrate a breakdown of the electric dipole approximation in the TPSE process where the interference between the 2ED and 2EQ transitions is important, as it increases the total rate by 63 %. Third, we explore platforms where entangled photons of different energy are emitted in the far-field in different directions. In the end, our framework is a complete tool to design emitters and nanostructures for the TPSE process, leading to a rich assortment of functional nanoantennas.

Non-Hermitian photonics attracted significant attention as a rising field in optics due to the emergence of numerous physical concepts and novel effects. Unlike systems described by a Hermitian Hamiltonian, where hermiticity ensures system closure to the environment and energy conservation, a non-Hermitian system enables the description of open systems and facilitates understanding of how a system can interact with the environment. We propose an innovative approach for simulating non-Hermitian dynamics by realizing a non-unitary photonic quantum walk, based on a light beam propagating in free space and manipulated via step operators acting jointly on its polarization and transverse momentum. We use the latter degrees of freedom to encode the coin and walker systems, respectively. To induce coin-rotation, we utilize a uniform liquid-crystal (LC) plate. An LC dichroic polarization grating is used instead to obtain a coin-dependent non-unitary translation operation on the walker. Through the combination of liquid crystals and dichroic absorbing dyes, we can manipulate both polarization and light amplitude. This development yields a compact and versatile platform that significantly expands the scope of photonic simulations in studying quantum dynamics. It introduces a new dimension for manipulating topological states, potentially enabling the observation of phenomena related to non-Hermitian topological phases.

Photonic circuits that can manipulate light in a unitary and reconfigurable way are promising candidates for optical processing of both classical and quantum information. However, engineering such circuits poses significant challenges in terms of minimizing losses, increasing the number of modes, and achieving multidimensional dynamics.

Here we present a novel photonic circuit based on spin-orbit photonics that, by realizing up to 20 timesteps of a two-dimensional quantum walk, couples a single input mode to hundreds of output modes. Our circuit consists of three liquid crystals metasurfaces that perform periodic and space-dependent polarization transformations on a light beam, effectively coupling circularly polarized spatial modes with different transverse momenta. These modes form the basis of a two-dimensional lattice where the quantum walk takes place.

We demonstrate the versatility and scalability of our circuit by operating it in different regimes and measuring the output modes distributions, which show high similarity with the theoretical predictions (>87%).